This invention is an improvement to procedures for the alkylation of isobutane by light olefins. More specifically, this invention relates to a procedure for the alkylation of isobutane by olefinic hydrocarbons in which a cut rich in normal butane is extracted from the reaction effluents, where said cut is upgraded by isomerization so as to produce isobutane intended for recycling in the alkylation unit.
The invention also relates to a device for implementing such a procedure.
As of 1930 the refiners developed alkylation procedures in which branched paraffins (or isoparaffins) react with olefins to create branched superior hydrocarbons, essentially intended to be incorporated in aviation gasoline, or to formulate high octane rating automobile gasoline.
The industrial conditions for implementation of the isobutane alkylation reaction by light olefins are well known to those skilled in the art. Indeed we can refer to the article by J. F. Joly, in the work entitled Procxc3xa9dxc3xa9s de Transformation (Le Raffinage du Pxc3xa9trole, Vol. 3, Chap. 7; Edition Technip 1998). In general, two charges, one containing isobutane and the other containing light olefins, are put in contact in an alkylation reactor, in the presence of a liquid or solid acid type catalyst. The reaction effluents are then treated in a fractionation area, in which the product sought, the alkylate, is separated through distillation in the form of a hydrocarbon mixture that contains for the most part 5 to 12 carbon atoms.
In a manner known in itself, the relative quantities of isobutane and olefins introduced in the alkylation unit make up a key parameter in the procedure""s implementation. Indeed, the isobutane/olefin ratio not only conditions the reaction""s yield, but also the quality of the alkylate that is produced. We know it is essential to use a large excess of isobutane compared with the olefins, in order to limit the secondary polymerization reactions of the olefins. This is why it is normal, in the industrial alkylation units, to operate with an isobutane-to-olefins molar ratio ranging between 3 and 25 based on the type of unit.
It is therefore necessary to have a large source of isobutane to feed the alkylation unit. Yet, isobutane is usually produced in limited quantities in refineries, unless, upstream from the alkylation, a unit such at those described in the article by G. L. Frischkorn, P. J. Kuchar and R. K. Olson, Energy Progress, 8(3), 154, 1988 that are specifically intended for the production of isobutane through isomerization of the normal butane is built, however this represents a costly investment.
Therefore, the quantity of available isobutane often limits the capacities of the alkylation industrial units. Consequently, we understand why it is customary to recuperate the excess isobutane that did not react in a reaction effluent fractionation column and to recycle it in the alkylation reactor.
It is also customary to separate the normal butane that is usually present in these effluents in the reaction effluent fractionation column. Indeed, the cut rich in normal butane thus obtained is directly amenable to beneficiation: it is usually incorporated in products such as gasoline or liquid petroleum gas (LPG).
A particularly advantageous practice consists in subjecting this cut rich in n-butane to an isomerization treatment which makes it possible to transform the normal butane into isobutane that can then be recycled in the alkylation unit. This solution, mentioned for example in the U.S. Pat. No. 5,675,052 is that much more interesting for the refiner as it makes it possible to better beneficiate the normal butane that is extracted from the effluents in order to produce an isobutane make-up intended to feed the procedure.
This solution also appears in the U.S. Pat. No. 5,565,617 that describes a fractionation system and a specific treatment adapted to the effluents emitted from an alkylation reactor in which the reaction is done in the presence of a solid acid catalyst and a halogen compound. This treatment, intended to eliminate halides that are present in the effluents, also incorporates, as an option, a step for drawing off a cut that contains normal butane, where said cut is isomerized before being recycled at the entry of the fractionation area of the alkylation effluents. Such an arrangement makes it possible to advantageously use the effluent fractionation area to separate the mixture of normal butane and isobutane that exits the isomerization area.
U.S. Pat. No. 4,324,937 describes a procedure for the production of gasoline type cuts from a mixture of propane and butane. This procedure includes a step for the production of isobutane through isomerization of the normal butane, a step for the production of propylene through dehydrogenation of the propane and a step for the production of superior hydrocarbons through alkylation of isobutane by propylene. The production of isobutane is guaranteed by isomerization both of the normal butane present in the starting mixture and that extracted from the effluents of the isomerization reaction. Optionally, the cut rich in n-butane can, before isomerization, be fractionated in a deisobutanizer whose function is to extract the residual isobutane from this cut, which makes it possible to move the balance of the normal butane isomerization reaction in favor of the production of isobutane.
However, so far the systems proposed in the prior art have not proven very satisfactory. In particular, despite the fact that they are potentially very advantageous, systems in which we isomerize then recycle a cut rich in n-butane extracted from the reaction effluents of the alkylation do not make it possible to reach the expected gains.
Indeed, if only one isomerization reactor is used, the yield from the conversion of normal butane to isobutane is often insufficient, and the quantity of the isobutane make-up produced by this system does not justify the investment linked to the installation and operation of the isomerization reactor.
One solution consists in using two isomerization reactors in series operating under different conditions of severity as is usually the case in the traditional normal paraffin isomerization units. However, this alternative, although it makes it possible to obtain a satisfactory yield for the isomerization of the recycling of the normal butane also doubles the investment since two reactors must be built and operated, therefore, in the end such a system not very profitable.
Continuing Applicants"" research in the alkylation area, Applicants looked into these systems where a cut rich in normal butane extracted from the reaction effluents is upgraded through isomerization, and Applicants have developed a procedure that makes it possible to remedy the inconveniences of the prior art.
In particular, Applicants have discovered that, surprisingly, the fact of previously extracting from said cut rich in n-butane the compounds with 5 or more carbon atoms that are present in it, considerably increases the yield of the normal butane isomerization reaction.
Indeed, Applicants put forward the hypotheses according to which the compounds with at least five carbon atoms that are created during the alkylation reaction and are present in a more or less important quantity in the cut rich in normal butane extracted from the reaction effluents, inhibited the isomerization reaction of the normal butane by behaving as catalyst poisons. Applicants also discovered that, by rigorously controlling the content of compounds with at least 5 carbon atoms (referred to hereafter as C5+ compounds) in the cut rich in n-butane, we were able to significantly increase the efficiency and therefore the profitability of the procedure.
Applicants have thus developed a procedure for the alkylation of isobutane by olefinic hydrocarbons in which a first hydrocarbon charge rich in isobutane is put in contact with a second charge of hydrocarbons rich in light olefins, under conditions that can trigger the alkylation of the isobutane by light olefins, the effluents emitted from the reaction area are treated in a fractionation column in order to extract at least a first cut rich in alkylate, a second cut rich in normal butane, and a third cut rich in isobutane, then said third cut is recycled at the entry of the alkylation reaction area.
This procedure is characterized by the fact that said second cut rich in normal butane is purified so as to lower its content in 5 or more carbon atom compounds to a value that is less than or equal to 5% by weight, said cut purified in this manner is treated in an normal butane to isobutane isomerization reactor, it is then recycled at the entry of the alkylation effluent fractionation column.
The procedure as set forth in this invention makes it possible to beneficiate the cut rich in normal butane extracted from the alkylation reaction effluents in the best possible way, as it makes it possible to produce, from this cut, a maximum quantity of isobutane intended as feedstock for the alkylation reactor. Indeed once free from almost all the C5+ compounds it contains, this cut is subjected to an isomerization treatment, whose yield is that much higher as said cut contains less C5+ compounds. The mixture of normal butane and isobutane thus obtained is then recycled at the entry of the column used for the fractionation of the alkylation reaction effluents. For that purpose, this mixture can either be combined, upstream from the fractionation column, with the alkylation reactor effluents, or directly injected into said alkylation reaction effluents fractionation column. The mixture of normal butane and isobutane is thus fractionated in a mixture with the reaction effluents: the isobutane that was produced in the isomerization reactor is drawn from the fractionation area in a mixture with the excess isobutane emitted from the alkylation, and all of it is recycled in the alkylation unit. As to the normal butane that was not isomerized in the isomerization reactor, it is drawn off mixed with the normal butane that is present in the alkylation reaction effluents and all of it is directed toward the C5+ compounds extraction system then toward the isomerization reactor.
Therefore, one major advantage of the procedure as set forth in the invention is that it makes it possible to substantially increase the quantity of the third cut rich in isobutane drawn from the reaction effluent fractionation column in order to be recycled toward the alkylation reactor. This gives the refiner the opportunity to either increase the alkylation unit""s production capacity, or, for a constant production capacity, to decrease the input of fresh isobutane in the alkylation reactor. The profitability of the unit is thus substantially increased.
Another advantage of the procedure as set forth in the invention is that is renders unnecessary the use of two isomerization reactors in series for the transformation of the normal butane extracted from the reaction effluents to isobutane. Indeed, once purified, meaning free of the C5+ compounds, said second cut rich in normal butane can be isomerized with a satisfactory yield in one single isomerization reactor. As the cost of building and operating a C5+ elimination system is much lower than the cost of a second isomerization reactor, the procedure as set forth in the invention offers an efficient solution, while limiting investments. Compared with the systems of the prior art, the procedure as set forth in the invention therefore makes it possible to produce, at a lesser cost and with optimal efficiency, a significant isobutane make-up intended to be used as feedstock for the alkylation procedure.
This invention also relates to a device that makes it possible implement the procedure described above.
Consequently, the object of the invention is an isobutane alkylation device using olefinic hydrocarbons that contains at least:
one alkylation reaction area, whose type is known in itself;
one fractionation column for the effluents that emanate from said reaction area, that contains at least three levels of draw off: a lower draw off level for a first cut rich in alkylate, a middle draw off level for a second cut rich in normal butane, and a level that is higher that the prior two levels, for drawing off a third cut rich in isobutane;
a means for recycling said third cut at the entry of the alkylation reaction area.
This device is characterized by the fact that it consists, of at least one means for selective extraction of said second cut rich in normal butane from the hydrocarbons with 5 or more carbon atoms, an isomerization reactor of normal butane to isobutane, and a means for recycling the second isomerized cut towards the entry of the fractionation column of the alkylation reaction effluents, all arranged in series.